As technology progresses, processes tend to proceed at a faster pace. Most processes require the coordination of a number of components, and a process can only proceed as fast as the slowest component allows unless multiple like components are used. There are certain processes in which the weight of an article is required, and great efforts have been made to obtain a scale that provides accurate, fast weighing. By accurate is meant the ability to weigh an object having a weight of up to 12 ounces within 1/32 of an ounce. By fast is meant the ability to weigh a stream of conveyed articles faster than one article per second. A process where there is a need for accurate, fast weighing is in the processing of mail. High speed systems have been developed whereby the appropriate number of inserts, which number may vary from envelope to envelope, are placed within an envelope. THe envelope is sealed and postage is printed on the envelope. Before the postage can be printed, however, it is necessary that the weight of the mail piece be determined.
In the past, weighing devices for such mail processing systems have been developed, but these generally have been rather slow. Actually, many prior weighing devices combined a standard scale with a mechanism that would stop the mail to allow weighing to take place.
Although these past weighing devices worked rather well with prior mail processing systems, with high speed inserters of contemporary design, the one function that had inhibited fast processing of mail was the weighing of mail pieces before postage is applied thereto. In order to overcome this problem, multiple scales would be used downstream from a high speed inserter and alternate mail pieces would be transported to such scales. Obviously, use of multiple scales is expensive and requires additional conveying apparatus that could result in a greater number of jams.
Certain prior high speed weighing devices employed to weigh articles that are part of a train of articles weighed each article while the scale was still in motion. See for example U.S. Pat. No. 3,800,893. The drawback to such a weighing system is that of expense. Still another scheme of fast weighing is to weigh a large number of articles simultaneously and obtain the average weight, but this is useless when the individual weight of each piece is required.
Recently a scale had been developed based on the principles of harmonic motion. Streams of articles can be transported individually unto the tray of the scale and the tray is caused to oscillate after an article has been placed on the tray. THe harmonic period of motion is measured and compared to a calibrated standard. Based on the comparison, the weight of the article is determined. The scale is referred to as a vibrating tray scale and is described in U.S. Pat. No. 4,778,018 and assigned to the assignee of the instant patent application.
Although the scale described in the U.S. Pat. No. 4,778,018 works well, it was found that improvement would be desirable in the locking mechanism for securing the tray of the scale while transporting articles onto the scale tray and during removal of an article following the weighing operation.